Re: [Qemu-devel] [PATCH] target-arm: Use common CPU cycle infrastructure

[Peter Maydell]

On 5 October 2015 at 15:27, Paolo Bonzini <address@hidden> wrote:
> I remember seeing a bug where this terrible implementation caused
> divisions by zero on the host.  The default implementation in
> include/qemu/timer.h should be changed to
> qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL).
>
> Note that in practice this terrible implementation is only used on
> ARM/AArch64.  Is PMCCNTR or something similar accessible from userspace?
>  I guess no, since even write access is enabled via PMUSERENR (and in
> general PMUSERENR ought to be false on a preemptive-multitasking OS).

Yeah, there's no guarantee on userspace access. I think the
fastest way to get some kind of count is to use a library
function that boils down to gettimeofday(2), which we will
do purely in userspace in the kernel VDSO if possible.

thanks
-- PMM